RU-486, the abortion pill pulled from the market in France on Wednesday, works by interfering with the action of a sex hormone, progesterone, that plays a central role in retaining a fertilized egg in the uterus.
The drug binds to the progesterone receptors on uterine cells, thus blocking the binding of progesterone molecules. Without progesterone, the endometrium that forms the inner lining of the uterus crumbles, causing bleeding, and the fertilized egg is expelled.
When taken within 49 days of a woman’s last menstrual period, RU-486 can cause the uterus to expel the fertilized egg in at least 80% of the cases, according to published research. The bleeding induced by the pill can last as long as 11 days.
Researchers who have worked with the drug say it can also be administered along with prostaglandin, a hormone that causes uterine contractions. Working together, the two treatments are 95% effective in causing abortions during the first trimester.